Hip joint deterioration frequently leads to patients undergoing total hip replacement (total hip arthroplasty). Such hip joint deterioration has many causes including, but not limited to, degenerative arthritis, rheumatoid arthritis, ankylosing spondylitis, avascular necrosis (both primary and secondary), osteoarthritis (both primary and secondary), ankylosis secondary to previous infections, spontaneous ankylosis, postsurgical ankylosis, bone tumors (both benign and malignant) around the hip joint, and hip fractures.
Approximately 193,000 total hip replacements were performed in the United States in 2002 (2002 National Hospital Discharge Survey, United States Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Health Statistics, May 21, 2004). Both the number of total hip replacements and the per capita rate of such operations have been increasing significantly.
During the decades that hip replacements have been performed, there have been many variations. The following description is for the typical, conventional hip replacement. No attempt is made here to include exhaustive detail of the conventional hip replacement or to describe the many variations. Such a hip replacement requires the removal of the head (ball) and neck of the femur. Then, the femur is reamed, and repeated broaching operations hollow out and clean approximately six inches of the interior of the proximal femur. The operations prepare the femur for the acceptance of a prosthetic implant that has a ball attached. The prosthetic implant stem portion is designed to fit the last broach that hollows out the upper interior of the femur. The worn cartilage and arthritic condition in the socket area in the hip is machined out to a hemispherical shape for the acceptance of a titanium alloy part that is secured in position with one or more screws. A plastic liner is then fit into the titanium part. Then, the ball on the prosthetic implant of the femur is inserted into the plastic socket prosthetic implant prepared in the hip bone.
There are a large number of variations in conventional stem-and-ball femur-side prosthetic implants. In some cases each prosthetic implant is a single piece; in some cases the “stem” and “ball” are two pieces which are attached. In some cases the prosthetic implant is cemented; in some cases it is not. Various materials have been used (e.g., various alloys, ceramics). Stem length in those prosthetic implants varies considerably. The specific geometries vary (e.g., curvature of the stem). Ball size of those prosthetic implants varies considerably.
Although total hip arthroplasty is major surgery, the operations are sufficiently common and usually successful that one could use the term “routine.” While the usual outcome of total hip arthroplasty is highly successful, the functional lifespan of the prostheses is often shorter than the life of the person into whom the prostheses are placed. The circumstance of people outliving their hip prostheses can result in additional surgeries to repair or replace the prostheses. Concerns about prostheses wearing out have been addressed numerous ways. Those include employment of materials and surface treatments intended to reduce wear. Attempts to prolong prosthesis life have also included employment of femur-side prosthesis equipped with a secondary bearing system in addition to the primary bearing system.
Veterinary total hip replacements are also performed. For animals with hip joints with similar mechanics to humans, the nature of the hip replacement procedure and the prosthetic implants used are similar to those used in humans.